The present invention concerns a belt drive, particularly for driving accessory parts of an internal combustion engine, of the type that uses a belt with multiple poly-V grooves.
The present invention has a preferred application in the transmission of movement between the internal combustion engine of a motor vehicle and a plurality of accessories of the engine comprising a reversible electric machine.
In the internal combustion engines of motor vehicles, to avoid installing a separate generator and starting motor, it is common practice to provide a single electric machine of a reversible type, commonly known as a starter/generator, which is able to act both as a generator, when it is activated by the internal combustion engine in normal running conditions to charge the battery of the vehicle, and as a starter, that is as an electric motor able to drag the internal combustion engine during starting.
In particular, movement is transmitted between the internal combustion engine and the starter/generator by means of a poly-V belt used for driving other accessories such as, for example, the compressor of the air-conditioning system and the water pump. The belt presents a plurality of continuous longitudinal ridges or ribs, defining V-shaped grooves between each other, and it cooperates with a pulley keyed onto the engine shaft and with a plurality of pulleys integral with the shafts of the respective accessories, and in particular with a pulley keyed onto the shaft of the starter/generator. The poly-V belt transmits movement by friction between the sides of the grooves and the sides of the corresponding ribs on the circumferences of the pulleys which engage the respective grooves.
In particularly damp conditions, there may be a strong reduction of the friction coefficient between the belt and the pulleys. In particular, tests carried out have revealed that, at starting, there may be slipping between the belt and the pulley of the starter/generator, which in this case has a drive function. The problem occurs mainly due to the high torque to be transmitted and due to the fact that the arc of winding the belt onto said pulley cannot be increased beyond certain limits for reasons of bulk and the layout of the drive.
Besides causing undesirable noises and premature wear of the belt, slipping between the belt and the pulley of the starter/generator can even prevent the starting of the internal combustion engine. It is therefore a problem that can seriously threaten the reliability of the motor vehicle.
U.S. Pat. No. 4,571,224 illustrates a belt drive system of the synchronous type, comprising a toothed belt and pulley. The pulley has continuous V-shaped circumferential ribs, shorter or of the same height as the teeth, which cooperate with corresponding longitudinal grooves on the teeth of the belt and having a depth shorter than or equal to the height of the teeth. Movement is transmitted through the meshing and coupling of circumferential ribs and grooves and has the function of preventing the lateral deviation of the belt.
This belt drive is therefore unsuitable to drive accessory parts of an internal combustion engine, particularly in the case where a reversible electric machine is used, in which the high torques to be transmitted require the use of a poly-V type belt.